This invention relates to methods for fabricating ceramic matrix composite (CMC) articles. More particularly, this invention is directed to a method for removing a core used to form a cavity in a CMC article.
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of iron, nickel and cobalt-base superalloys. However, components formed from superalloys must often be provided with some form of thermal and/or environmental protection in order to exhibit adequate service lives in certain sections of a gas turbine engine, such as the turbine, combustor and augmentor.
Silicon carbide (SiC)-based ceramic matrix composite (CMC) materials have been proposed as materials for certain components of gas turbine engines, such as the turbine blades and vanes. Various methods are known for fabricating SiC-based CMC components, including melt infiltration (MI), chemical vapor infiltration (CVI) and polymer pyrolysis (PIP) processes. Though these fabrication techniques significantly differ from each other, each involves the use of tooling or dies to produce a near-net-shape part through a process that includes the application of heat at various processing stages. As with turbine blades and vanes formed of more conventional superalloy materials, CMC blades and vanes are preferably equipped with cooling passages and holes in order to reduce their operating temperatures. Cooling passages and holes, as well as other cavities, are typically formed in CMC components using a combination of removable and expendable tooling. The external contours of hollow CMC components are typically formed using removable tooling that can be reused in most cases. Internal cavities can also be formed using removable tooling, though conventional silica (SiO2) and alumina (Al2O3) cores widely used with investment casting methods have also been used.
Silica and alumina cores require removal with a leaching compound, including salts, hydrogen fluoride (HF) and alkalis such as sodium hydroxide (NaOH) and potassium hydroxide (KOH). In some cases, the exposed surfaces of a metal investment casting are coated with a masking material to prevent surface attack by the leaching compoundxe2x80x94the internal surfaces of the casting cannot be masked due to the presence of the core. As a result, the critical external surfaces of the casting are protected, while less critical internal surfaces are subject to mild attack by the leaching compound. However, leaching compounds conventionally used to remove silica cores from investment castings aggressively attack many CMC materials, and particularly those that contain silicon and boron, typically in the form of SiC and boron nitride (BN), respectively. Accordingly, attempts to remove silica cores from CMC components susceptible to attack by leaching compounds suffer unacceptable attack of its internal surfaces, which reduces the structural integrity of the CMC component.
Accordingly, it would be desirable if a method were available for fabricating a CMC component with an internal cavity formed by a core that can be readily removed without damaging the walls of the cavity.
The present invention generally provides a method of removing a core from a CMC component in which the core was used to form an internal cavity. According to one embodiment of the invention, the core is removed from a partially or fully densified CMC component by heating the component and core to a temperature at which the core deteriorates but below a temperature at which the component would melt or otherwise be damaged. In this embodiment, the core is preferably formed from a carbon-based material, an elemental silicon-based material, a ceramic body coated with an elemental silicon-based material, or a particulate ceramic material bonded together with an elemental silicon-based material.
According to a second embodiment of this invention, the component is only partially densified, and then impregnated with a coating material that is resistant to a leaching compound that is suitable for removing the core. The core can then be removed using the leaching compound without damage to the internal surfaces of the component defined by the core when the component was fabricated. In this embodiment, partial densification of the component permits infiltration of the coating material to the internal surfaces, which would otherwise be subject to aggressive attack by the leaching compound.
In view of the above, it will be appreciated that the present invention enables CMC components to be fabricated with one or more internal cavities without the surfaces of the cavities being damaged during the removal of the core or cores used to form the cavities. Accordingly, this invention enables precision internal cavities to be formed in a CMC component, such as cooling passages in a CMC turbine blade or vane.
Other objects and advantages of this invention will be better appreciated from the following detailed description.